Simulation of multiwavelength conditions in laser picosecond ultrasonics

International audienceComplete numerical simulations are given under SciLab® and MATLAB® coding environments, concerning propagative acoustic wavefronts, for laser picosecond ultrasonics under multiwavelength conditions. Simulations of the deformation field and its propagation into bulk material are given under different wavelength configurations for optical pump and probe beams, which are used to generate and to detect the acoustic signal. Complete insights concerning the dynamics of the acoustic waves are given, considering the absence of carrier diffusions into the material. Several numerical approaches are proposed concerning both the functions introduced to simulate the wavefront (Heaviside or error) and the coding approach (linear/vectorized/Oriented Object Programming), under the pure thermo-elastic approach. © The Author(s) 2021

Mueller matrix polarimetry for improved liver fibrosis diagnosis

International audienceAn experimental Mueller matrix polarimeter is used to quantify human liver fibrosis by measuring retardance and depolarization of thin biopsies. The former parameter is sensitive to fibrillar collagen, the latter is specifically sensitive to fibrillar collagen around blood vessels, which is not significant for liver fibrosis diagnosis. By using depolarization like a filter, retardance distribution enables to distinguish between disease stages and limit the high degree of observer discrepanc

Competition between inverse piezoelectric effect and deformation potential mechanism in undoped GaAs revealed by ultrafast acoustics

By using the picosecond ultrasonics technique, piezoelectric effect in <111> GaAs undoped sample at both faces (A[111] and B[-1-1-1]) is experimentally studied. We demonstrate that piezoelectric generation of sound can dominate in <111> GaAs material over the deformation potential mechanism even in the absence of static externally applied or built-in electric field in the semiconductor material. In that case, the Dember field, caused by the separation of photo-generated electrons and holes in the process of supersonic diffusion, is sufficient for the dominance of the piezoelectric mechanism during the optoacoustic excitation. The experimental results on the sample at both faces reveal that in one case (A face), the two mechanisms, piezoelectric effect and deformation potential, can compensate each other leading to a large decrease of the measured Brillouin oscillation magnitude

Twisted ferroelectric liquid crystals dynamic behaviour modification under electric field: A Mueller matrix polarimetry approach using birefringence

A low frequency and high amplitude rectangular voltage V has been applied during different increased duration to Twisted Surface Stabilized Ferroelectric Liquid crystal (TwFLC) samples in which the alignment layers of the two substrates were rubbed along two different directions between 0°0° and 90°90°. The optical bistability properties have been evaluated using the specific Mueller Matrix formalism that allows a simultaneous access, through a single-shot measurement, to different polarimetric coefficients. In this new approach, the ellipticity ?R?R and the azimuthal ?R?R polarimetric parameters, extracted from the birefringence Mueller Matrix MR will be considered in priority. Several significant parameters, such as the horizontal offset ?V?V, the degree of asymmetry DA, the characteristic area S of the hysteresis loop, are used to characterize the degradation observed into the hysteretic behaviour of the samples, for different values of ?, at different duration T of exposure to V, before reaching the so-called stripes regime, giving a new experimental point of view concerning the evolution of the dynamic properties of the samples studied. The ?R(V)?R(V) and the ?R(V)?R(V) hysteresis loops are specifically examined. Static mapping related to ?R(T)?R(T) is given too. Among the different possible physical origins of the observed degradation, the in-plane anchoring energy contribution will be particularly examined, and a theoretical model is proposed that also gives access to different physical parameters, through a new approach